Those figures don't surprise me at all cos the fact is those lovely virus-writing people by and large don't target Linux OS's cos they're free. I understand that the Mac OS is now also free, possibly as Apple hardware costs an arm and a leg.
Microsoft is still seen as public enemy number one by a great many people (that's another debate altogether) and therefore is constantly targeted. It makes sense, therefore, that Microsoft, through sheer necessity, should have the least vulnerabilities.